1. Field of the Invention
The present invention concerns digital transmission systems, and in particular the transmission of data by Time Division Multiplex (TDM). In TDM multiplex several data streams are combined into a single stream for more economical transmission. The combining of the data streams is carried out by a multiplexer.
2. Description of Related Art
The basic format for transmitting multiplexed digital data is commonly known as a frame. Essentially a frame is a fixed-length group of bits comprised of a specified series of digits known as a Frame Alignment Word together with data bits and other control bits. This specified series is repeated at intervals which define the frame length. The Frame Alignment Word enables a demultiplexer to align itself to the data stream. Since the multiplexer assembles the bits from its inputs in a known order, for example by bit interleaving, alignment at the demultiplexer permits correct reconstruction of the input or tributary signals. The series of bits which form the Frame Alignment Word need not be consecutive within a single frame. Thus a Frame Alignment Word can be either bunched (consecutive bits) or distributed within a frame or even a combination of the two. Generally a bunched word gives more rapid alignment. A potential source of error in the reception of TDM data is that the receiving equipment becomes out of step with the received data. This condition is known as loss of alignment.
The ability to regain alignment quickly on a data stream containing an alignment word or words is an important criterion in many digital systems.